Re: downloading

Your download will still be using some of your upload capacity as well. Packets will be sent to acknowledge the receipt of the packets you are downloading, so that if any go missing then they are automatically resent (with TCP at least).

As for the latency, the pings need to be sent from your browser (uploaded) to the the speedtest server, which will then reply with a message (downloaded) back to the client. Since the download is saturated, and the upload has reduced capacity, that would delay them.